Powerful mechanical forces are required for introducing solids into liquid media. This depends to a large extent on the ease with which the solid can be wetted by the surrounding medium and on the affinity to this medium. To reduce these dispersing forces, it is customary to employ dispersing agents which facilitate incorporation. These are in most cases surface-active substances, also known as tensides, which have an anion-active or cation-active and nonionic structure. These substances are added in relatively small quantities, either by direct application to the solid or by introduction into the dispersing medium. The effort required for dispersion is substantially reduced by such a tenside.
It is also known that these solids tend to reagglomerate after the dispersion process, thus vitiating the effort previously expended for dispersion and leading to serious problems. This phenomenon is explained by London/van der Waal's forces by which the solids attract each other. To overcome these forces of attraction, it is necessary to apply adsorption layers on the solids. This is achieved by using such tensides.
During and after dispersion, however, an interaction between the solid particle and the surrounding medium takes place and desorption of the tenside occurs, accompanied by its replacement by the surrounding medium, which is present at a higher concentration. This surrounding medium, however, is in most cases not capable of building up such stable adsorption layers, and the whole system breaks down. This manifests itself by a rise in viscosity in liquid systems, loss of gloss and shift in colour tone in lacquers and coatings, insufficient development of colour power in pigmented plastics, and decrease of mechanical strength in reinforced plastics.
European patent application 154,678 discloses a dispersing agent comprising the reaction product of a polyisocyanate having a valence of from 2.5 to 6 with a monohydroxyl compound, a difunctional component and a compound containing a basic ring nitrogen.
Due to the nature of the compounds and the reactions connected therewith, it is essential for the product to be manufactured by a multistage method. With regard to the production efficiency this is not desirable. Moreover, the problem occurs that both the intermediate and the end product are not very stable and tend to gelatinization.
More in particular, it may be noted that it is necessary according to the known method to use a multistage process wherein after stage 1, in which the polyisocyanate must first be reacted with the monomer compounds, a difunctional compound cannot be used until in stage 2. If this uneconomical process is not used and the components from stages 1 and 2 are therefore reacted from the beginning, complications will occur in process stage 3 to the extent of a substantial formation of gel particles and a gelatinization of the entire mass.
The intermediate resulting from reaction stages 1 and 2, which is reacted in stage 3 with compounds of formula III to obtain a ready-for-delivery product, only has a limited shelf life (about 24 hours) and is therefore not suitable for supply as a basis for further addition reactions.
The products manufactured by the process described show a marked tendency towards gelatinization in the third reaction stage so that these reactions can only be effected in highly diluted solutions, resulting in end products which only have a relatively low solid content. The increase in solid by subsequent vacuum distillation is not possible in view of the marked tendency towards coagulation.
The known dispersing agents further have a compatibility, a solubility and a dispersing activity which are not very good.
It is an object of the present invention to provide dispersing agents which, as compared with the known dispersing agents, have improved properties, in particular improved compatibility with binding agents, improved solubility and improved dispersing activity.
It is further an object of the present invention to provide dispersing agents which do not have the above disadvantages with regard to their preparation or to a substantially less extent, and which particularly result in dispersions of solids which do not tend, or only to a minor degree, to reagglomerate after the dispersion process.